The invention concerns a tire with radial carcass reinforcement anchored in each bead to at least one bead wire and comprising a crown reinforcement consisting of at least two so-called working plies, superposed and formed by parallel wires or cords in each ply and crossed from one ply to the next by forming angles with the circumferential direction of the tire, the absolute value of which can range between 10xc2x0 and 60xc2x0.
International application WO 97/30857, with a view to lowering the operating temperature of a heavy truck-type tire with radial carcass reinforcement while providing a solution that is both effective and economical, recommends adding to said carcass reinforcement a crown reinforcement comprising at least two working crown plies of inextensible cords, crossed from one ply to the next by forming angles of between 10xc2x0 and 45xc2x0 with the circumferential direction, characterized in that an axially continuous additional ply, made up of wire elements, oriented roughly parallel to the circumferential direction, is positioned radially between the working plies, said additional ply having an axial width that can be equal to at least 1.05 times the axial width of the widest working crown ply.
In that context, the additional ply can be made up of so-called semielastic continuous steel wire cords, that is, cords with relative elongations at rupture exceeding 2%. Those cords make it possible to obtain a level of rigidity suitable for harmonious distribution of the circumferential tension between the working crown plies and the additional ply. Said cords are advantageously described as xe2x80x9cdouble-modulus,xe2x80x9d that is, presenting a tensile stress/relative elongation curve having shallow slopes for low elongations and a roughly constant and steep slopes for high elongations. The very low modulus before curing, for elongations less than 2%, make possible an increase in the circumferential development of the additional ply during curing of the tire.
The additional ply can also be made up of steel wire cords circumferentially oriented and cut so as to form sections much shorter than the circumferential length of the ply, the gaps between sections being axial offset from one another. Such an embodiment makes it possible to endow the additional ply with any desired rigidity.
The choice of elastic or cut cords for reinforcement of the additional ply does not afford the best fatigue strength of said ply, owing to a reduction of breaking load of the elastic cords, as well as to the existence of stress concentrations in the calendering mix of the cut cords.
French patent application FR 97/09996 shows that better fatigue strength of the additional ply of circumferential wire reinforcement elements, combined with easier industrial production, can be obtained thanks to the use of corrugated wire elements as reinforcing elements in the plane of the ply, placed radially between the working plies, the corrugations of the reinforcing elements being parallel to each other, in phase and oriented parallel to the circumferential direction, and the ratio of linear rigidity of the additional ply to the sum of linear rigidities of the other plies of the crown reinforcement being equal at most to 0.10.
Japanese patent application JP 02/081708 describes a crown reinforcement consisting of two corrugated reinforcing elements, the ratio of amplitude of the corrugations to the wavelength of the same corrugations being less at the edges than at the center of the reinforcement. Said ratio makes possible a greater rigidity at the ply edges and thus avoids a radial elevation of the tread edges.
Considering the progress achieved in durability of the treads for heavy truck tires, as well as in endurance of the crown reinforcement, the endurance of the crown reinforcements needs to be improved from the standpoint of resistance to separation between carcass reinforcement and crown reinforcement, resistance to separation between the working crown plies and fatigue strength of the circumferentially oriented cords of the additional ply.
The tire with radial carcass reinforcement, according to the invention, having a crown reinforcement comprising at least two working crown plies made up of inextensible wire reinforcing elements, crossed from one ply to the next, and an axially continuous additional ply, made up of steel wire reinforcing elements, corrugated in the plane of the ply, being placed radially between the working plies and having an axial width L32 at least equal to 50% of the maximum axial width of the carcass reinforcement, is characterized in that the reinforcing elements are steel, corrugated in the plane of the ply and of greater diameter than that of the reinforcing elements of the plies of the crown reinforcement situated radially on both sides of said additional ply, the corrugations of the elements being parallel to one another, in phase and oriented parallel to the circumferential direction, so that the ratio of amplitude of wavelengths of the corrugations of corrugated elements decreases axially from the center to the edges of said ply, reaching a minimum at said edges.
Corrugated wire elements whose mean direction is circumferential are elements whose corrugations have axes in a direction forming an angle with the circumferential direction within the span of xc2x15xc2x0 with respect to 0xc2x0.
Likewise, the ratio of amplitude to wavelength of the corrugations will preferably range between 2% and 15%, the amplitude of a corrugation being by definition measured crest to crest. The ratio of amplitude to wavelength, ranging between 2% and 15%, makes it possible to have reinforcing elements not interfering with shaping of the tire blank in the vulcanization mold, while making it possible, after curing, mounting and inflation of the tire, to obtain the crown reinforcement linear rigidity necessary for improvement of the endurance of said crown reinforcement.
The additional ply can have an axial width equal to at least 1.05 times the axial width of the widest working crown ply.
The additional ply of corrugated circumferential elements, in the particular case of tires of H/S shape ratio equal to at least 0.50, can be used in combination with a first working crown ply, the meridian curvature of which is roughly equal to the meridian curvature of the subjacent carcass reinforcement, so as to be able to place it parallel to said carcass reinforcement without interposed sections. The additional ply is then provided with a roughly zero curvature on being separated from the first working ply by appropriate sections of roughly triangular shape.
The crown reinforcement, according to the invention, will advantageously be completed by a so-called protecting crown ply made up of straight elastic steel wire cords, oriented relative to the circumferential direction at an angle roughly equal to the angle formed by the cords of the radially outermost working crown ply and the axial width of which can be either less or greater than the average width of the working plies.